Injection moulding machinery

ABSTRACT

A shuttle is mounted in a passage connecting ducts from injection barrels. An outlet in the passage wall connects to a common sprue channel. The shuttle isolates one duct from the opening at each end of its travel but at no time blocks the opening. The shuttle is preferably a ball and is preferably located in the spigot of a shut-off tap.

This invention relates to injection moulding machinery and in particularto an injection unit for injecting plastic materials sequentially into amould cavity from separate sources.

By injecting plastic materials sequentially into a mould cavity fromseparate sources, articles may be formed having a skin of the materialfirst injected enclosing a core of the second injected material.

Various embodiments of this process and machinery suitable therefor aredescribed in, inter alia, our British Patent Specification Nos.1,156,217, 1,219,097 and 1,255,970.

In order to achieve rapid moulding cycles it is desirable that the wallsof the mould cavity into which the materials are injected are maintainedat such a temperature as to enhance solidification of the syntheticresin material. The mould temperatures are conveniently those utilisedfor the conventional injection moulding of the skin material, or, whereone of the materials is thermosetting, the temperatures normallyutilised for injection moulding of that thermosetting material. Thuswhere both the synthetic resin materials are thermoplastics that aresolidified by cooling, the mould walls are preferably maintained at atemperature below the softening point of the skin material e.g. at atemperature in the range 0° to 100°C by water or oil circulating withinthe mould members defining the mould cavity. Where one or both of thesynthetic resin materials are thermosetting i.e., materials that areconverted to the solidified state by heating, the mould walls arepreferably heated to the temperature necessary to effect solidificationof the thermosetting resin.

Except where the skin material is thermoplastic and the corethermosetting in which case a heated mould would be utilised to effectcuring of the core material, on injection of synthetic resin materialinto the mould cavity, the material adjacent to the walls solidifiesbefore that in the centre of the cavity. As the material is injectedinto the mould cavity, it contacts the mould wall and then spreads outfrom the injection orifice, herein termed the sprue passage, towards themould extremities. Thus while material is beiing injected, the meltfront is continuously moving until it is halted by contact with themould wall. We have noticed that if the melt front temporarily stopsmoving before the mould cavity is filled, a line may appear on thesurface of the moulded article at a point corresponding to the positionof the melt front when it was temporarily halted. This line is believedto be caused by preferential solidification of the melt at the pointwhere the melt front is stationary. Some materials exhibit this line,herein termed a hesitation mark, to a greater extent than othermaterials.

During the injection cycle it is necessary to inject differentmaterials, i.e., the skin material and then the core material, and so itis necessary to switch from injection of one material to another. Whileit is possible to arrange that the switchover is very rapid neverthelessa hesitation mark is still liable to occur.

In some cases it may be possible to arrange that injection iscontinuous, for example by arranging plugs of the materials to beinjected in series in an injection barrel of an injection mouldingmachine. While this may avoid hesitation marks, it is less versatilethan systems using separate injection barrels as the same injectionconditions, e.g. temperatures, have to be used for each material andalso contamination of the skin material by core material is possible.

In our British Pat. No. 1,291,111 we described a shuttle valve systemwhich enabled injection of the second material to commence beforeinjection of the first material stopped. By arranging for this "overlap"of injection the hesitation marks can be reduced or eliminated. We havenow devised an alternative form of such a valve.

Accordingly we provide an injection unit of an injection mouldingmachine, for injecting plastic materials sequentially into a mouldcavity with injection of the second material commencing beforecompletion of injection of the first material, comprising

1. two injection barrels,

2. a single channel for connection with a sprue passage of a mouldcavity,

3. ducts connecting each injection barrel with the single channel,

4. means to force plastic material from each barrel into the mouldcavity via said ducts, channel, and sprue passage,

5. isolator means to isolate both injection barrels from the spruepassage at the same time, and

6. selector means having dispositions wherein each barrel in turn isconnected to the single channel with the other barrel isolatedtherefrom, and in the transitional disposition of said selectror means,between said dispositions wherein one barrel is connected to the singlechannel and the other barrel is isolated therefrom, both barrels areconnected to the single channel,

characterised in that said selector means is located at the junction ofsaid ducts and said single channel and comprises

a. a passage each end of which connects to one of said ducts,

b. an opening in the wall of the passage, intermediate said ends,connecting with said single channel, the width of said opening beingless than the width of said passage,

c. a shuttle mounted in said passage and being moveable, under theaction of the pressure exerted by the plastic material, along the lengthof the passage, said shuttle blocking the passage, at each extremity ofits movement, to prevent flow of material from the duct at that end ofthe passage, via the passage, to the opening, said shuttle at no timecompletely blocking said opening so that at any position of the shuttle,one or both of the ducts is connected to the opening.

The shuttle is thus of such a configuration that, if it has a part ofthe same cross-section as the passage, the length of that part, in thedirection of movement of the shuttle, is less than the greatestdimension of said opening in the direction of movement of the shuttle.

The shuttle is preferably a ball and the passage is of circularcross-section having the same diameter as the ball. Alternatively shortcylindrical shuttles can be used, or shuttles of other cross-sections.The shuttle preferably has a distance of travel greater than, or equalto, the greatest dimension of the opening in the direction of movementof the shuttle.

In operation, when one material is being injected, the injectionpressure of that material emerging into the passage from the duct at oneend thereof forces the shuttle to its extremity of its travel on theside of he opening in the passage remote from said one end. In thisposition the shuttle seals or isolates the other duct from communicationwith the opening.

When injection of material from the first barrel is nearing completion,the pressure on the material in the second barrel is increased. Theshuttle is thus forced to move towards the other end of the passage,thus connecting the duct from the second barrel to the opening. As theshuttle at no time completely blocks the opening, material can thus flowfrom both injection barrels, via their associated ducts into the passageand hence into the mould. When the shuttle reaches the other extremityof its travel the duct from the first barrel is isolated from theopening, thus stopping flow of material from the first barrel.

The isolator means may be either upstream or downstream of the shuttleselector valve. It is preferably upstream of the shuttle selector valveso that both of the injection barrels can be isolated from the passagein the shuttle selector valve. This enables both injection barrels to berecharged with plastics material independently of each other.

In a preferred form the isolator is a rotary tap and particularly is atap arranged both upstream and downstream of the shuttle selector valve.This may be achieved by locating the shuttle selector valve inside thespigot of a rotary tap. Such a tap spigot has two inlets one forcommunication with each duct, and a single outlet, which corresponds toaforesaid single channel, for communication with the sprue passage. Onrotation of the spigot from the "on" position, the spigot inlets aretaken out of communication with the ducts, thereby isolating each barrelfrom the sprue passage and from each other, upstream of the shuttleselector valve, and also the outlet of the spigot is taken out ofcommunication with the sprue passage.

One embodiment of the invention is now described with reference to theaccompanying drawings wherein

FIG. 1 is a diagrammatic layout of the apparatus,

FIG. 2 is a partial cross-section through the tap part of the injectionunit showing the shuttle at one extremity of its travel;

FIG. 3 is a view corresponding to part of FIG. 2 showing the shuttle inmid-position;

FIG. 4 is a view similar to FIG. 3 but showing an enlarged opening inthe shuttle passage;

FIG. 5 is a section along the line V--V of FIG. 4 with the shuttleomitted; and

FIG. 6 is a view, similar to FIG. 3 but showing an alternative form ofshuttle at one extremity of its movement.

In FIG. 1 there is shown an injection unit comprising two injectionbarrels 1, 2 each provided with reciprocating screw injection rams whichcan be rotatably driven by motors 3 and reciprocated by hydrauliccylinders 4. The outlets of the injection barrels are connected via atap 5 to a sprue passage 6 of a mould assembly 7. The tap 5 can berotated between "on" and "off" positions by means of a motor 8.

The tap arrangement is shown in more detail in FIGS. 2 to 4.

Referring to FIGS. 2 and 3 the rotary tap 5 comprises a rotatable spigot9 mounted in a housing 10. A squared projection 11 on one end of spigot9 enables the spigot to be rotated by the drive motor 8.

The two outlets from the injection barrels 1, 2 are shown as hot runners12, 13. The housing 10 also provides the start of the sprue passage 6.The tap spigot 9 is provided with inlet ducts 14, 15 and a singlecylindrical outlet channel 16. In the "on" position shown in FIG. 2 eachinlet duct 14, 15 is connected to its associated hot runner 12, 13respectively, and the outlet channel 16 is connected to sprue passage 6.On rotation of spigot 9 by means of motor 8 driving squared projection11, the ducts 14, 15 and channel 16 can be moved out of communicationwith hot runners 12, 13 and sprue passage 6 respectively, thusconstituting an "off" position wherein both injection barrels areisolated from each other and from the sprue passage 6.

Inside spigot 9, duct 15 leads into one end of a cylindrical boredisposed with its longitudinal axis along the axis of rotation of spigot9.

A plug 17 is screwed into an enlargement of the cylindrical bore andthis plug 17 has a cylindrical projection 18 which fits inside the bore.Duct 14 connects with a duct 19 in projection 18 and thence with acylindrical passage 20 formed by the plugged cylindrical bore. Thispassage 20 has an opening 21 disposed in the wall thereof which openingforms the start of channel 16. The diameter of opening 21 and channel 16is less han the diameter of passage 20. It is thus seen that passage 20connects duct 15 with duct 14 via the duct 19 which forms an extensionof duct 14.

Situtated in passage 20 is a shuttle consisting of a ball 22. This ballhas the same diameter as passage 20 and is movable therealong. By virtueof the fact that the shuttle is here a sphere, the length of theshuttle, in the direction of movement thereof, that has thecross-section of passage 20 is the thickness of the circumference and sois actually of infinitesimal length.

This length is thus less than the greatest dimension of opening 21,i.e., the diameter thereof, in the direction of movement of the shuttle.At each end of its movement, the ball seats against a tapered surfaceconstituting the junctions of ducts 15 and 19 with passage 20. At oneextremity of the movement of ball 22 (as shown in FIG. 2), its centreline, and hence the circumference in contact with the walls of passage20, lies at one side of opening 21 while at the other extremity ofmovement, it lies to the other side of opening 21.

In the extreme position, as shown in FIG. 2, material can be injectedfrom hot runner 12 to sprue passage 6 via, in turn, duct 14, duct 19,passage 20 and channel 16. At the same time the pressure on the materialbeing injected forces ball 22 firmly against the tapered seatingsurface, thus isolating duct 15 from passage 20 and hence hot runner 13from sprue passage 6.

In the transitional position as the ball moves from one extremity ofmovement to the other, as shown in FIG. 3, both duct 19 and duct 15 arein communication with passage 20 and because ball 22 can at no timeblock opening 21, material can flow from both hot runners 12 and 13 tothe sprue passage 6.

The ball 22 is moved from one position to the other by the pressureexerted thereupon by the plastic material. When the injection ofmaterial from hot runner 12 is nearing completion (i.e., with theshuttle in the FIG. 2 position), the pressure on the material in theother injection barrel, and hence in hot runner 13, is increased.Because the whole of the cross-sectional area of the ball 22 issubjected to the pressure of the material in passage 20, while the areaof the ball subjected to the pressure of material in duct 15 is somewhatless, due to the diameter of seating of the ball being less than thediameter of passage 20, the material in duct 15 has to be subjected to ahigher pressure than that prevailing in passage 20 before the ball 22will start to move. As the ball moves, material will be forced into thesprue passage 6 from both hot runners. When the injection ram in theinjection barrel 2 connected to hot runner 12 stops moving forward theapplied pressure will drop and ball 22 will seat against the tapered endof projection 18, thus isolating hot runner 12 from the sprue passage 6.

If it is desired to have a greater degree of overlap of injection,opening 21 may be enlarged in the direction of movement of the shuttle(although the width of the opening 21, i.e., in the directionperpendicular to the direction of movement, must remain less than thediameter of passage 20 to stop ball 22 dropping into the opening 21).Such an enlargement is shown in FIGS. 4 and 5.

Another advantage of this arrangement we have found is that the secondinjected material tends to be injected fairly concentrically down thesprue passage and hence the mould cavity is filled evenly.

Instead of a ball shuttle, alternative forms may be used. One of theseis shown in FIG. 6. Here the passage 20 is of rectangular cross-sectionand the shuttle 22 has a middle portion 23 of corresponding rectangularcross-section. The length of this middle portion is less than thelongitudinal dimension of the opening 21 so that shuttle 22 does notclose opening 21 during its path of movement. The remainder of thelength of shuttle 22 is of lesser cross-section and the ends are taperedto seat against the tapered ends of passage 20.

The sequence of operation of the apparatus of the present invention willgeneraly be as follows, referring to the apparatus of FIGS. 1 to 3 withthe tap spigot in the "off" position, the mould cavity 7 is closed, theinjection barrels 1, 2 are charged with plasticised plastics material byscrew plasticisation in the normal way and the `skin` material injectionbarrel 1 (connected to hot runner 12) pressurised. The tap is thenturned to the "on" position and the skin material injection ram forcedforwards by its associated hydraulic cylinder 4. The pressure of thematerial in hot runner 12 forces ball 22 to the FIG. 2 position, if itwas not there already, and skin material is injected, via sprue passage6, into the mould cavity. When injection of skin material is nearlycomplete, the core material injection ram in the injection barrel 2connected to hot runner 13 is forced forwards; this pressurises thematerial in duct 15 until ball 22 starts to move. Injection of skinmaterial continues meanwhile and injection of core material commences assoon as ball 22 is moved clear of the seating at the end of passage 20.Movement of the skin material injection ram is halted and the pressureexerted on ball 22 by the core material from duct 15 causes the ball 22to seat against the tapered end of projection 18, thus stoppinginjection of skin material. Often, when injection of core material isnearly complete, the skin material injection ram is forced forward torepressurise the skin material in duct 19 and move ball 22 back to theFIG. 2 position as the core material ram is halted, thus injecting afurther amount of skin material. The tap is then rotated to the "off"position, the moulding allowed to solidify and the injection ramsrecharged with plasticised material. After solidification the mouldingis removed from the mould cavity, together with the sprue formed insprue passage 6 and the system is then in readiness for the nextmoulding cycle. The further injection of skin material enables anyresidual core material to be swept from the channel 16, so that oncommencing injection of skin material in the next moulding cycle, nocore material is present in channel 16, which being inside the tapspigot cannot be removed with the sprue, and so contamination of theskin injection in the next moulding cycle is avoided. It also ensuresthat the shuttle 22 is moved to the FIG. 2 position in readiness for thenext moulding cycle.

It will be appreciated that if the length of travel of the shuttle isincreased, the shuttle may lift off its seating before opening 21 isbrought into communication with the portion of passage 20 between thepreviously isolated duct and the shuttle. In this case no injection ofmaterial from that previously isolated duct will occur until the shuttlemoves sufficiently to permit communication between that duct and opening21.

I claim:
 1. An injection unit of an injection moulding machine, for injecting plastics material sequentially into a mould cavity with injection of the second material commencing before completion of injection of the first material, comprisinga. two injection barrels; b. ducts connecting with each injection barrel; c. a passage, each end of which connects to one of said ducts, said passage having an opening in its wall, intermediate said ends, the width of said opening being less than the width of said passage; d. a single channel for connection of said opening with a sprue passage of a mould cavity; e. means to force plastics material from each barrel into the mould cavity via said ducts, passage, channel and sprue passage; f. isolator means, having an on-position for communicating the single channel with the injection barrels and an off-position wherein both injection barrels are isolated from the single channel at a location upstream of said passage; and g. a shuttle located in said passage and being moveable, under the action of the pressure exerted by the plastics material, along the length of the passage, said shuttle having a cross-section substantially the same as the passage and at each extremity of its movement blocking the duct at that end of the passage to prevent flow of material from that duct to the single channel, and said shuttle at no time completely blocks said opening so that, when the isolator means is in the on-position, depending on the position of the shuttle, one or other or both of the injection barrels are connected to the single channel via said ducts, passage and opening.
 2. An injection unit as claimed in claim 1 wherein the passage is of circular cross section and the shuttle comprises a ball of the same diameter as said passage.
 3. An injection unit as claimed in claim 1 wherein the shuttle has a distance of travel at least equal to the greatest dimension of the opening in the direction of movement of the shuttle.
 4. An injection unit as claimed in claim 1 wherein the isolator means, when in the off-position, also isolates the single channel from the sprue passage.
 5. An injection unit as claimed in claim 4 wherein the isolator means is a rotary tap and the passage and shuttle are located inside the spigot of the tap. 